State that most genetic mutations have no effect on the phenotype, some…

The idea

A genetic mutation is a change in the base sequence of DNA. The phenotype is the observable characteristic an organism shows. Most mutations change nothing observable: the genetic code is degenerate, so a changed base often still codes for the same amino acid, and many changes sit in non-coding DNA. Some mutations have a small effect, for example altering one amino acid so a protein works slightly less efficiently and a characteristic shifts only a little. Rarely, a single mutation has a significant effect, such as a change that produces a non-functional protein or a new characteristic. So the size of a mutation's effect on phenotype is not fixed by the mutation alone; it depends on where the change occurs and what the protein does.

What earns the marks

• Most genetic mutations have no effect on the phenotype, because the code is degenerate or the change lies in non-coding DNA.
• Some mutations have a small effect on the phenotype, for example a slight change in a protein's function.
• Rarely, a single mutation significantly affects the phenotype, for example producing a non-functional protein or a clearly new characteristic.
• The effect depends on the location and consequence of the base-sequence change, not simply on the fact that a mutation occurred.
• A mutation is a change in the DNA base sequence; the phenotype is the observable characteristic.

Common mistakes and how to put them right

• Assuming every mutation changes the phenotype.

  Put it right by stating that most mutations have no observable effect.

• Thinking a mutation always causes disease.

  Put it right by noting that many mutations are neutral and only rarely does one mutation have a significant effect.

• Confusing mutation with phenotype.

  Put it right by defining mutation as a change in DNA base sequence and phenotype as the observable characteristic.

• Believing the effect size is random.

  Put it right by linking the effect to where the change occurs and what the protein does.

Exam tips

• Read the question stem for the words 'most', 'some' and 'rarely' and match the frequency to the correct effect size.
• Eliminate options that claim all mutations change the phenotype before choosing.
• Link a significant-effect example to a protein becoming non-functional rather than to a vague idea of 'damage'.